This invention relates to electric switches and in particular to electric switches which operate a secondary set of contacts in response to a welded condition of a primary set of contacts. Still more particularly, the invention relates to switches of the aforementioned type which are pushbutton operated, and to such switches which have an additional set of contacts particularly adapted for completing a dynamic braking circuit to a load controlled by the switch.
A welded contact in a switch maintains a current path through the switch after the switch has been operated to an off condition. This condition can be surprising to the user of apparatus controlled by the switch, and could be dangerous. If, for example, the switch controls a motor of an electric driven vehicle, a welded contact could cause the vehicle to continue to be propelled after the switch is operated to an off condition in expectation of stopping the vehicle.
U.S. Pat. No. 4,647,727 issued Mar. 3, 1987 to C. G. Sontheimer discloses a switch having normally closed auxiliary contacts operable to an open condition upon a welded condition of main switch contacts. The auxiliary contacts represent a separate complete switch within the switch housing having its own actuator and being operated by a specific condition of the linkage and operator of the main switch. The auxiliary switch also has its own terminals separate from the main contact terminals and therefore only provides safety in the system being controlled if the contacts are appropriately connected into the system.
U.S. Pat. No. 4,216,358 issued Aug. 5, 1980 to J. Brozille discloses a pushbutton snap switch wherein a pair of movable contacts oscillate between spaced pairs of stationary contacts by a snap action over-center mechanism. In the event the contacts remain welded upon depression of the pushbutton to drive the actuating mechanism over-center, continued depression of the pushbutton physically rotates the stationary contacts to break the circuit at a different location on the stationary contact. This weld-responsive action occurs on depression of the pushbutton which is normally associated with actuation of the switch, not release thereof. Release of the pushbutton permits the faulty contacts to be reclosed and the circuit to be re-energized.